Understanding Surkhi in Lime Mortar
Surkhi is a finely powdered brick aggregate, typically made from burnt bricks. When added to lime mortar, it significantly alters its properties. Traditional lime mortar, made from fat lime (calcium oxide or hydroxide), hardens slowly by absorbing carbon dioxide from the air (carbonation). This process is slow, especially in damp conditions or thick sections, and plain fat lime mortar does not set under water.
The Role of Surkhi: Enhancing Mortar Properties
The primary reason for adding Surkhi to lime mortar is its pozzolanic activity. Pozzolanic materials are siliceous or alumino-siliceous substances that react with calcium hydroxide (released during the slaking and carbonation of lime) in the presence of water to form cementitious compounds. These compounds contribute to the strength, durability, and, importantly, the setting properties of the mortar.
Why Surkhi Provides Hydraulic Properties
Adding Surkhi imparts 'hydraulic properties' to the lime mortar. Hydraulic properties refer to the ability of a binder (like mortar) to set and harden by chemical reaction with water, even under water or in damp environments. This is crucial for construction below ground level, in foundations, or in areas exposed to moisture where carbonation alone is insufficient or impossible.
The chemical reaction involves the silica and alumina in Surkhi reacting with the calcium hydroxide ($\text{Ca(OH)}_2$) from the slaked lime:
$\text{Calcium Hydroxide} + \text{Reactive Silica (in Surkhi)} + \text{Water} \rightarrow \text{Calcium Silicate Hydrates (CSH)}$
$\text{Calcium Hydroxide} + \text{Reactive Alumina (in Surkhi)} + \text{Water} \rightarrow \text{Calcium Aluminate Hydrates (CAH)}$
These CSH and CAH compounds are similar to those formed during the hydration of Portland cement and are responsible for the hydraulic set and increased strength.
Analyzing the Options
- hydraulic properties: As discussed, Surkhi reacts with lime in the presence of water to form compounds that cause the mortar to set and harden even in damp conditions or under water. This makes the mortar hydraulic. This option aligns with the function of pozzolans like Surkhi.
- adhesive properties: While the overall adhesion of the mortar to bricks or stones is important and is influenced by the binder, the primary role of Surkhi addition is not specifically to enhance the adhesive properties compared to other fundamental binding properties. Adhesion depends on various factors including the binder type, mix consistency, and surface preparation.
- solubility properties: Surkhi is a stable, insoluble material. Its addition does not increase the solubility of the mortar; in fact, the formation of insoluble calcium silicate and aluminate hydrates makes the hardened mortar less soluble and more durable, especially in wet environments.
- cohesive properties: Cohesion refers to the internal bond strength within the mortar itself. While the formation of cementitious compounds by Surkhi reaction does increase the overall strength and cohesion of the hardened mortar compared to plain lime mortar, the most significant property imparted that plain lime mortar lacks is the ability to set hydraulically.
Therefore, the most accurate answer describing the main property furnished by adding Surkhi to lime mortar is hydraulic properties.
Revision Table: Properties of Lime Mortar
| Property | Plain Fat Lime Mortar | Fat Lime Mortar with Surkhi |
|---|---|---|
| Hardening Mechanism | Carbonation (reaction with $\text{CO}_2$ from air) | Carbonation + Hydraulic reaction (with water) |
| Setting Environment | Requires air; slow in damp/thick sections | Sets in air and water/damp conditions |
| Strength Gain | Slow and gradual | Faster initial strength gain, higher ultimate strength |
| Durability | Less durable in wet conditions | More durable, especially in wet conditions |
| Shrinkage | Higher shrinkage on drying | Reduced shrinkage |
Additional Information: Pozzolans and Lime Mortar
Surkhi is a common example of a natural or artificial pozzolan. Other pozzolanic materials used with lime include volcanic ash (like the original pozzolana from Pozzuoli, Italy), fly ash (a byproduct of coal combustion), silica fume, and ground granulated blast furnace slag.
The use of pozzolans with lime dates back to ancient times (Romans used volcanic ash). This practice allows for the creation of durable binders suitable for structures exposed to water, such as aqueducts, baths, and harbor walls. The reaction with pozzolans effectively utilizes the calcium hydroxide, which is a soluble component in hardened plain lime mortar and can be a source of weakness or leaching.
By providing hydraulic properties, Surkhi makes lime mortar more versatile and suitable for a wider range of construction applications than plain fat lime mortar alone.
Important Questions from Miscellaneous-Engineering
- Which one of the following lists identifies the wastes from common manufacturing and industrial process, such as solvents that have been used in cleaning or degreasing operations?
When once a pocket of smoke, containing air pollutants, is released into the atmosphere from a source like an automobile or a factory chimney, it gets dispersed into the atmosphere into various directions depending upon the
1. prevailing winds
2. temperature
3. pressure conditions
Select the correct answer.
During the compaction test, the weight of compacted soil specimen along with mould is 38.2 N. The volume and weight of mould are 0.95×10-3 m³ and 20.5 N respectively and the water content is 12%. The dry unit weight of the compacted specimen will be nearly
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